Module 14

Updated: 06/24/2008

Lenses: Some

Final Elements

You may recall that the inside of a lens -- a zoom lens in particular -- is packed with many glass elements.  When light makes its way through the glass into the camera, each element reflects light off its shiny surface.

Even if each one reflected only five percent of the light hitting its surface, which is not unusual for glass, no light at all would get to the camera. This, of course, would defeat the purpose of the lens. Fortunately, there is a solution.

    Lens Coatings

To reduce the problem of internal reflections, the surface of each element is covered with a micro-thin, antireflection coating. This lens coating typically gives the elements a light blue appearance and significantly reduces the amount of light lost due to surface reflections.

This means that in a zoom lens, such as the one shown here, the front and back of each of the more than twenty glass elements will have antireflection coatings.

Although lens coatings are much more resilient than they used to be, they're still relatively easy to scratch permanently. One or more bad scratches on a lens diminishes both sharpness and image contrast.

Because of the way lenses are manufactured, it's generally less costly to replace the lens than to try to repair it.

Since it's easy for  an object to come in contact with a camera lens, remember to use a lens cap when you're transporting the camera and, in fact, anytime you're not using it.

A lens cap not only guards against scratching, but also keeps off dirt and fingerprints, which can also reduce sharpness and contrast.

Some lens caps are made of white translucent plastic designed to replace the white cards used to white balance a camera. If you put the capped lens in the dominant light source and push the white balance button, the camera will white balance on the color of the light coming through the lens cap.

Although this is a quick way to color balance a camera, as we'll later see, it's not as accurate as zooming in on a carefully positioned white card.

Cleaning Lenses

Small quantities of dust on a lens will not appreciably affect image quality, but fingerprints and oily smudges are a different matter. Not only do they reduce image sharpness, but if not promptly removed, the acids in fingerprints can permanently etch themselves into a lens coating.

However, each time you clean the lens, you increase the risk that tiny abrasive particles picked up by the cleaning tissue will create microscopic scratches in the coating. For this reason, you should not just routinely clean your lens; do so only when you see dirt or dust on its surface.

To clean a lens, first remove any surface dirt by blowing it off with an ear syringe or by brushing it off with a clean camel's hair (extremely soft) brush.

If this doesn't remove the dirt, dampen a lens tissue with lens cleaner, and very gently rub the lens in a circular motion. Turn or roll the tissue slightly to avoid rubbing any dirt over the lens surface.

Never drip lens cleaner directly on a lens. It can easily seep behind lens elements and create a major problem. And don't clean a lens with silicon-treated lens tissues or the silicon-impregnated cloths commonly sold for cleaning eyeglasses. The residue may permanently discolor the coating.        

Condensation On the Lens

Condensation and raindrops on a lens can distort or even totally obscure an image.

When a camera moves from a cool to a warm area, the lens frequently fogs up. This can be a major problem in cold climates.

Even though you wipe moisture off the lens, the lens may continue to fog up until its temperature equals the surrounding air.

Condensation can also take place within a camcorder and cause major problems. For this reason, many camcorders have a dew indicator that detects moisture or condensation and shuts down the unit until the moisture evaporates. A message such as "dew" will typically display in the viewfinder. To reduce the effect of condensation when bringing a camcorder in from the cold, you should allow thirty minutes or so for the camcorder to reach room temperature.

By the way, laptop computers can have the same problem -- especially if they are stored in the trunk of a car overnight in freezing temperatures and then brought into a warm room. There is no "dew indicator," of course; they may just refuse to boot.  As in the case of camcorders, allowing the unit to slowly warm up for thirty minutes or so in warm, dry air should fix the problem.

Rain Jackets

Although manufacturers discourage use of video cameras in rain, snow, and wind-driven sand or dust, we must often shoot news stories under such conditions.

Camera rain jackets, such as the one shown on the right, cover all but the viewfinder and the very end of the camera lens.

Or, in an emergency, you can use a plastic garbage bag. Just cut holes for the lens and viewfinder, and then use rubber bands to secure the plastic around each. Basic camera controls should be operational though the plastic bag, which is much easier if the bag is transparent.

Most camcorders contain many delicate moving parts, and just a bit of dirt, sand, or moisture in the wrong place can put the unit out of commission.    

Shot Boxes

In studio work you'll often use a set sequence of shots on a regular basis. Wide-shots, two-shots, and one-shots in a newscast are good examples.

Shot boxes are electronic lens controls that memorize a series of zoom lens positions, complete with zoom speeds and focus settings.

Note the series of white buttons shown here. The camera operator can program each button for a particular shot. This approach adds speed and consistency to studio work.

Image Stabilizers

 In 1962, a lens mechanism was introduced that compensated (within limits) for camera vibration and unintentional camera movement. Called an image stabilizer, the first model was a gyroscopically controlled mechanism that resisted short, fast movements by shifting lens elements in the opposite direction. 

Things have advanced significantly since then, and today the simplest, digital stabilization, is totally electronic -- it "floats" an active picture frame within a slightly larger one.

As the camera moves, the smaller frame shifts within the larger target area in an attempt to compensate for the movement.

If, for example, the camera moves slightly to the right, the digital frame will electronically move in the opposite direction, canceling the movement on the camera's target. Many consumer grade camcorders use this approach. 

Although this electronic image stabilization approach has seen some major technical improvements in recent years, the reduction in the size of the usable target image area still tends to sacrifice image resolution and clarity.

Professional videographers prefer optical image stabilization.

Optical image stabilization uses two parallel, floating optical surfaces within the lens that act as a kind of flexible prism.

These optical surfaces electronically detect the camera's movement, and the voltage that's generated as a result changes the configuration of the prism. This alters the angle of light passing through the prism and shifts the image on the target in the opposite direction. Since the full target image is used, no loss of image quality occurs.

As you might assume, this approach is more complex and costly, which is why you don't see it on consumer-grade camcorders.

With all types of stabilizers, the camera operator must learn "to compensate for the compensation." In panning from left-to-right, for example, a short delay occurs as the camera tries to compensate for the move. But once beyond a certain point, the stabilizer can't compensate for the movement and the image starts to move as intended.

At the end of the pan, however, the image may continue to move for a moment until the system comes back into balance. This means the camera operator may have to end the pan a moment early and allow the camera to complete the move.

Today, many "high-end" image stabilizalizers use sophisticated fiber optic servo devices.  This technology can cancel vibration from a helicopter or a moving vehicle.

The GyroCam helicopter mount shown on the left above not only compensates for vibration, but can also be completely controlled (e.g., pan, tilt, zoom, iris) from within the helicopter. Pilots use this type of device to follow fugitives and car chases on the ground.      

Lens Mounts

Many types of video cameras, especially consumer-type cameras, have zoom lenses permanently mounted to the camera body, and the lens can't be removed. Some video cameras, however, allow you to change lenses to meet specific needs. With these, you can either unscrew the lens (in the case of C-mount lenses) or turn a locking ring (in the case of the bayonet mounts).

C-Mounts

With a camera using a C-mount, the lens screws into a finely threaded cylinder about 25mm in diameter.

The C-mount was the first type of lens mount used with small video cameras because it takes advantage of a wide array of 16mm motion picture camera lenses.

Today, it's primarily industrial video cameras, including closed-circuit surveillance cameras, that use C-mount lenses.

Bayonet Mounts

Most professional video cameras use some type of bayonet mount. It's easier to use than the C-mount, because you can remove the lens without going through many rotations.

B4 Lens Mounts

Professional video cameras with a 2/3-inch or 1/2-inch chip (imaging device) commonly use a B4 lens mount.

35mm Lens Mounts

The primary consumer camcorder that uses interchangeable lenses is the Canon XLO1. It uses a bayonet mounting system that accepts Canon's extensive array of 35mm still camera lenses. Another manufacturer makes a video camera adapter for lenses designed for Nikon cameras.

Three Categories of Video Camera Lenses

We can classify the lenses used on video cameras into three categories:

• Studio/field lenses are completely enclosed in a metal housing that includes the focus and zoom motors, as well as sensors for the external controls.  
  
  

• ENG/EFP camera lenses are lightweight and have the controls mounted on the lens. They also feature a macro, or extreme close-up mode, and often a 2X focal length extender that doubles the effective focal length at all zoom settings.
    
• Electronic cinematography lenses are available in zoom or prime and are designed to accept film camera accessories. They typically have large focus, iris, and zoom scales and incorporate both motorized and manual controls.
  
  
• Rather than f-stops, the iris settings on these lenses are often calibrated in the similar, but somewhat more accurate, T-stops. (T-stops are based on the actual light transmission of the lens at various openings and not simply on the iris opening diameter formula.)
  
  
  Because different lenses vary in light transmission -- even at the same f-stop -- T-stop settings are more consistently accurate when used with different lenses. (The "T" in T-stop is capatalized, but not the "f" in f-stop.)